In the wet process method of producing phosphoric acid, phosphate rock and sulfuric acid are reacted under conditions which result in the formation of reaction products, phosphoric acid and gypsum, in accordance with the chemical equation: EQU Ca.sub.10 (PO.sub.4).sub.6 F.sub.2 +10H.sub.2 SO.sub.4 +20H.sub.2 O=10CaSO.sub.4.2H.sub.s O+6H.sub.3 PO.sub.4 +2HF
As this equation indicates, the major mineral usually present in most conventional phosphate rocks is a fluoride containing species which yields hydrofluoric acid during the reaction. The hydrofluoric acid then reacts with active silica normally present in the rock to give hydrofluosilicic acid according to the following general chemical equation: EQU 6HF+SiO.sub.2 =H.sub.2 SiF.sub.6 +2H.sub.2 O
This latter reaction is of practical importance industrially since the normally very corrosive hydrofluoric acid is transformed into the less corrosive hydrofluosilicic acid.
Phosphate rock in which a significant amount of fluoride is replaced by chloride, such as in rock containing the mineral fluochlorapatite, or phosphate rock deficient in active silica can present very serious corrosion problems if it is used to manufacture phosphoric acid by the wet process method. This corrosion problem is associated with the presence of free hydrochloric and/or hydrofluoric acids dissolved in the phosphoric acid-gypsum reaction slurry. Efforts to remove the chloride contaminant economically from phosphate rock containing fluochlorapatite have been unsuccessful because the chloride is insoluble and tightly locked within the crystal lattice in what is known as channel sites.
Some phosphate ores contain other troublesome mineral species. Iron containing minerals such a hematite, Fe.sub.2 O.sub.3, and geothite, HFeO.sub.2, and aluminum containing minerals such as crandallite, CaAl.sub.3 (PO.sub.4).sub.2 (OH).sub.5.3H.sub.2 O, will dissolve in the reaction slurry during the manufacture of phosphoric acid by the wet method, yielding a phosphoric acid which is usually considered unsuitable for further processing to fertilizer products if the level of iron or aluminum contamination is too high. Generally phosphoric acid having an (Fe.sub.2 O.sub.3 +Al.sub.2 O.sub.3) to P.sub.2 O.sub.5 ratio much greater than about 0.095 is considered unsuitable for the production of commercial fertilizers. Efforts to remove these contaminants from the phosphate rock before it is used for phosphoric acid manufacture generally have been only partly successful. Removal of iron and aluminum contaminants from the phosphoric acid product is often practiced, but the methods employed have resulted in major losses of phosphorous values in a waste or "raffinate" stream. This waste stream normally is of little economic value, since it is usually unsuitable for use in the manufacture of a conventional fertilizer product.
Finally, other minor mineral contaminants which occur in some phosphate rocks may cause serious problems during the manufacture of phosphoric acid by the wet process. For example, the hard mineral ilmenite, FeTiO.sub.3, can cause severe abrasion to processing equipment. Manganese minerals, such as pyrolusite, MnO.sub.2, contain manganese in a high oxidation state; phosphate rock containing such species can yield highly oxidative phosphoric acid which, under certain conditions, can be severely corrosive. Since these minerals rarely occur in the phosphate rocks normally used in the manufacture of phosphoric acid, methods of using phosphate rock containing these species have not been developed.
In the past, large sedimentary deposits of high grade phosphate rock have been available for the manufacture of phosphoric acid by the wet process method. Because of the relatively uniform nature of these deposits, and because they are contaminated by relatively innocuous minerals such as dolomite, silica sand and various clays, their beneficiation and use in the manufacture of phosphoric acid is well understood.
As these sedimentary deposits are mined out, the more unconventional phosphate rock deposits such as those containing fluochlorapatite, iron and aluminum mineral species will have to be exploited. Accordingly, a need exists for processes for producing and purifying phosphoric acid by the wet process without undesirable corrosive effects from these more unconventional and untapped phosphate rocks.